Been working on a Northstar MD40 flaker with a Bohn condensing unit (404) running two Copeland compressors. When I arrived the unit was off on oil pressure switches. No oil in comps or reservoir. Filled comps with oil. The unit ran but comps pumped oil out to system and no oil returned. Checked oil filter - ok, checked oil separator - ok. Finally checked for oil at the evaporator and I was able to pull out a five gallon bucket of oil before I had to leave (im sure there is more). The oil returns off the bottom of the unit and is supposed to run through a heat exchanger, which uses heat from liquid line to boil refrigerant from oil. The installing contractor did not install a heat exchanger.. Instead, the oil line comes out of the bottom of the evaporator rises to an elevation about three feet above the top of the ice maker and then stabs into the side of the suction line. I guess they were thinking that the refrigerant/liquid would enter suction line and be separated at the accumulator. That is all fine and well but I don't understand how the oil would ever be able to flow from the bottom of the exchanger (suction pressure) to the suction line which is at the same pressure. There is a sight glass in the oil return line and I do see oil in the glass but no movement. I checked for restrictions and didn't find any from the evaporator to the suction line. Anybody ever seen an ice flaker installed this way? Any help would be appreciated.

10-03-2012, 07:31 AM

icemeister

I believe your problem may be due to the location of the oil return tap on this flooded evaporator. The bottom return makes me think this unit is designed for ammonia, because the "Freon" units I've seen have had skimmer taps on the side at the liquid refrigerant level. The difference is with NH3 the oil tends to accumulate at the bottom of the refrigerant and with other refrigerants it floats to the top.

10-03-2012, 09:14 AM

tranger03

Icemeister,

I thought the exact same thing; however, when I contacted North Star directly they informed me that they use the same model for both ammonia and halocarbon. Given the differences in miscibility that you already stated, I can't see why the manufacturer wouldn't build the unit specific to the refrigerant used.

10-03-2012, 09:27 AM

icemeister

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I may be mistaken about the oil return location for North Star. (Perhaps I was thinking of a different manufacturer.) Anyway...I found a copy of their service manual and it states the halocarbon units use the same oil return location as the ammonia units...a 1/2" tap near the bottom of the evaporator. :whistle: (See p. 3)

Apparently then, solution would be to either to increase the rate of the liquid/oil mix in the return or decrease the rate of oil circulation...ie, a better oil separator.

They say the high circulation of refrigerant in their evaps results in a fairly even distribution of oil in the mix, but my guess is this only works if the rate of oil in circulation is relatively low...meaning very good oil separation is needed.

10-03-2012, 01:23 PM

tranger03

Thank you very much for the service manual. Everything that I had told the customer was backed up in the manufacturer literature. Really appreciate the help.

10-03-2012, 06:33 PM

icemeister

Quote:

Originally Posted by tranger03

Thank you very much for the service manual. Everything that I had told the customer was backed up in the manufacturer literature. Really appreciate the help.

You may want to call them and discuss your problem. I'm thinking the oil separator you have is a standard type which doesn't have the separation efficiency needed for these flakers. A coalescent or helical type separator would be a big improvement.

10-03-2012, 10:03 PM

tranger03

Believe me, icemeister, I have been in contact with the manufacturer. The installation was done by another contractor with used equipment. You are right about the condensing unit having only a conventional oil separator on it (which does not appear to be factory installed). This being just one of a few other piping problems lead to zero oil return. The customer was looking for answers and it was great to be able to justify my recommendations with literature from the manufacturer. Sounds like you've worked on these ice machines quite a bit. This was the first of its kind that I had seen. Sort of trial by fire but fun and also challenging. Thanks again for your help.